IR Spectroscopy

Question 1

What colour is IR light?

Answer 1

RED

Question 2

Range of IR we are interested in?

Answer 2

2500- 5000nm

Question 3

What is wave number?

Answer 3

V = 1/ wavelength(cm)

Question 4

Relation between energy and V ?

Answer 4

Higher the V, more the energy

Question 5

Do molecules excite after absorbing IR?

Answer 5

NO- IR energy too low- molecules remain in ground stage after absorbing

Question 6

When is a molecule IR active?

Answer 6

Must be a change in dipole moment as a result of the vibration that occurs when IR is absorbed

Question 7

Why does it show a broad bend instead of sharp peaks?

Answer 7

Each vibration is associated with several rotational levels- so broad bend produced rather than sharp peaks

Question 8

IR spectra x axis and y axis?

Answer 8

Y- %T

X- V (4000- 650cm-1)

Question 9

Absorption of IR is shown through PEAKS- True or False?

Answer 9

FLASE- shown by troughs

Question 10

Do bonds vibrate all the time?

Answer 10

YES

Question 11

Do molecules bend and stretch at certain frequencies?

Answer 11

YES

Question 12

Frequency at which a given bond vibrates is constant - T or F?

Answer 12

TRUE

Question 13

In mid IR region- molecule can go from V1 to V2 directly - T or F?

Answer 13

FALSE- can only go from Vo —> V1, V1–> V2

Question 14

Two different molecules can have the same IR spec- T/F

Answer 14

FALSE- no two molecules can have same IR spec- that’s why used for comparison and ID

Question 15

What are the two types of vibrations?

Answer 15

Stretching

Bending

Question 16

Two types of stretching?

Answer 16

Symmetrical

Asymmetrical

Question 17

Types of in- plane BENDING?

Answer 17

Scissoring

Rocking

Question 18

Types of out of plane BENDING?

Answer 18

Wagging

Twisting

Question 19

Asymmetric occur at higher f than symmetric - T/F

Answer 19

TRUE- asymmetric requires more energy

Question 20

Are all vibrations IR active?

Answer 20

NO- only those that result in a net change of polarity - dipole moment

Question 21

Correlation between vibration and abs?

Answer 21

Vibrations that have bigger dipole moment—> stronger absorption

Question 22

Are symmetrical bonds IR active?

Answer 22

NO- no dipole moment- no IR peaks

Question 23

How to find out if a molecule has a dipole moment?

Answer 23

If centre of the +be charge is different to centre of -ve charge

Question 24

More electronegativity difference = bigger dipole moment intensity?

Answer 24

TRUE

Question 25

Does bending require less energy than stretching?

Answer 25

YES- takes more energy to excite stretching vibration than bending
Higher V- shorter wavelength- higher F

Question 26

To absorb IR, a molecule MUST…?

Answer 26

• have a dipole change during vibration

- have molecular asymmetry by stretching and or bending

Question 27

Absorbance of IR of a bond depends on which 3 things?

Answer 27

1- mass of atoms connected to bond- if heavier, harder to stretch
2- dipole moment
3- strength of bond- stronger bond need more energy to vibrate- harder to stretch